Electron microscopes are used to image objects on a very small scale, to overcome limitations in light microscopes. For example Scanning Electron Microscopy (SEM) involves imaging an object of interest by generating a demagnified beam of electrons at an object and detecting the interactions. Interactions may take the form of “secondary electrons” from inelastic interaction, “backscatter electrons” from elastic interactions, x-rays and Auger electrons. Other signals that maybe detected include plasmons, phonons, bremsstrahlung radiation, and cathodoluminescence.
In an SEM type microscope an electron gun emits and accelerates a beam of electrons, which is demagnified by a series of lenses. The demagnefied beam is progressively scanned across the object in a regular pattern, also known as raster scanning. A detector is then used to detect the interactions to form an image. The intensity information from the detector may be translated to a grey scale array of pixels on a display, with brightness corresponding to higher signal intensity.
Generally electromagnetic or electrostatic fields are used to scan and demagnify the beam. For example a condenser lens (often a system of several lenses adjusted by a single control) is used as the primary means of reducing the beam diameter produced by the electron gun. The condenser lens is usually between the electron gun and a series of apertures. An objective lens near the sample demagnifies the beam to the smallest diameter at the surface of the object.
In order to scan the beam across the object non-concentric electromagnetic or electrostatic fields are used to deflect the electron beam. For example one or more scan coils are usually located near or within the objective lens assembly. Because the scan coils can control the position of the beam on the object they may also be used for determining magnification of the image and electronic shifting of the imaged area.
The objective lens (using electric fields, magnetic fields, or a combination of both) demagnifies the primary beam on to the specimen. An ideal lens would demagnify the primary beam of electrons into a perfect point at the specimen, an infinitely small probe, however, in practice, lens aberrations limit the probe to be a finite spot. Chromatic aberration is dominant among these aberrations, which arises from the fact that there is a small range of kinetic energies in the primary beam, called energy spread.
Electron guns inherently involve energy spread, and even cold field emission electron guns have a typical spread of 0.2 eV. Thus in prior art SEMs chromatic aberration results from the objective lens demagnifying the electrons with slightly different energies at slightly different focal positions. This has the effect of defocusing the beam, which distorts the image of the object. Chromatic aberration is one of the main limits on image resolution for SEM, and is greater for lower beam landing energies.